Process for producing vinyl chloride polymer

ABSTRACT

There is provided a process for producing a vinyl chloride-based polymer, in which a suspension polymerization of either vinyl chloride monomer, or a mixture of vinyl chloride monomer and another copolymerizable monomer, is conducted in a polymerization vessel fitted with a reflux condenser, the process including the steps of: (A) adding to the reaction mixture a high-activity, oil-soluble polymerization initiator, with a 10-hour half life temperature of no more than 40° C. at a concentration of 0.1 mol/L in benzene, for a specified time within a period from the commencement of heat removal using the reflux condenser through to completion of the polymerization, (B) adding water either continuously or intermittently to the reaction mixture through the supply pipe for the polymerization initiator, at least during the period from the commencement of addition of the high-activity, oil-soluble polymerization initiator through to completion of that addition, and (C) passing steam through the supply pipe following completion of the addition of the water. According to the present invention, the process for producing a vinyl chloride-based polymer by aqueous suspension polymerization in a polymerization vessel fitted with a reflux condenser can be improved, the heat removal capability can be utilized effectively to shorten the polymerization time, and a high quality vinyl chloride-based polymer with an extremely low level of fish eyes can be produced.

TECHNICAL FIELD

The present invention relates to a process for producing a vinylchloride-based polymer, and more particularly to a process for producinga vinyl chloride-based polymer by suspension polymerization whichenables the stable production of a high quality polymer with minimalfish eyes, at a high level of productivity.

BACKGROUND ART

Suspension polymerization of either vinyl chloride monomer, or a mixtureof vinyl chloride monomer and another copolymerizable monomer, isgenerally conducted in the following manner. A polymerization vesselfitted with a reflux condenser, a jacket, and a cooling baffle or coilor the like is charged with an aqueous medium, a dispersant (asuspension agent), and a polymerization initiator, as well as variousother additives, as required. Subsequently, hot water is passed throughthe jacket to increase the temperature inside the polymerization vesselto a predetermined polymerization reaction temperature, therebyinitiating polymerization. By subsequently passing cold water throughthe jacket, baffle, coil, and reflux condenser to remove the heatgenerated by the polymerization reaction, the polymerization reaction isallowed to proceed with the temperature inside the polymerization vesselmaintained at a predetermined reaction temperature.

In recent years, attempts have been made to improve productivity byshortening the polymerization time. One method of shortening thepolymerization time involves increasing the quantity of polymerizationinitiator added, thereby increasing the polymerization reaction rate.However, the ability to remove heat from the polymerization vessel islimited, and the quantity of polymerization initiator can only beincreased within the bounds allowed by this heat removal capability. Asa result, there are limits to how much the polymerization time can beshortened using this method.

One example of a proposed process that enables better shortening of thepolymerization time is a process in which, by combining a high-activitypolymerization initiator and a low-activity polymerization initiator,the reaction rate can be controlled at both the initial stages ofpolymerization and the closing stages of polymerization, therebyreducing the polymerization time (patent reference 1). According to thisprocess, the heat removal capability of the polymerization vessel can beutilized effectively in the initial stages and closing stages of thepolymerization, but in the middle stages of the polymerization, the heatremoval capability is excessive, and can not be effectively utilized.This tendency is particularly marked in those cases where a refluxcondenser is used as one of the heat removal devices.

Another proposed process that enables further shortening of thepolymerization time is a process in which a high-activity, oil-solublepolymerization initiator is added in the period following commencementof heat removal using the reflux condenser, through until a certainpolymerization conversion rate is reached (patent reference 2).According to this process, the heat removal capability of thepolymerization vessel can be used effectively, and the vinylchloride-based polymer can be produced with good productivity. Howeverin this process, the high-activity, oil-soluble polymerization initiatoradded during the polymerization requires some time to disperse uniformlythrough the reaction mixture inside the polymerization vessel, meaningthe concentration of the polymerization initiator develops an unevendistribution, which causes an increase in the number of fish eyes in theproduct polymer. Furthermore, because the high-activity, oil-solublepolymerization initiator is added to the polymerization vessel,containing the unreacted monomer, over a long period during thepolymerization, polymer scale tends to adhere to the inside of, and theregion around the outlet of, the supply pipe for the high-activity,oil-soluble polymerization initiator. As a result, the pipe cansometimes become blocked. Furthermore, because the adhered scale canbreak away and become incorporated within the polymer, the level of fisheyes within the product tends to increase.

Patent Reference 1: Japanese Laid-open publication (kokai) No. Hei6-166704

Patent Reference 2: Japanese Laid-open publication (kokai) No. Hei7-82304

DISCLOSURE OF INVENTION

Problems Invention Aims to Solve

Accordingly, an object of the present invention is to improve theprocess for producing a vinyl chloride-based polymer by suspensionpolymerization of either vinyl chloride monomer, or a mixture of vinylchloride monomer and another copolymerizable monomer, in apolymerization vessel fitted with a reflux condenser, thereby providinga process for producing a vinyl chloride-based polymer which enables theheat removal capability to be utilized effectively and thepolymerization time to be shortened, and which yields a high qualitypolymer with extremely low levels of fish eyes.

Means for Solution of the Problems

In order to achieve this object, the present invention provides aprocess for producing a vinyl chloride-based polymer, wherein asuspension polymerization of either vinyl chloride monomer, or a mixtureof vinyl chloride monomer and another copolymerizable monomer, isconducted in a polymerization vessel fitted with a reflux condenser, theprocess comprising the steps of:

(A) adding to the reaction mixture a high-activity, oil-solublepolymerization initiator, with a 10-hour half life temperature of nomore than 40° C. at a concentration of 0.1 mol/L in benzene, for aspecified time within the period from commencement of heat removal usingthe reflux condenser through to completion of the polymerization,

(B) adding water either continuously or intermittently to the reactionmixture through the supply pipe for the polymerization initiator, atleast during the period from commencement of addition of thehigh-activity, oil-soluble polymerization initiator through tocompletion of that addition, and

(C) passing steam through the supply pipe following completion ofaddition of the water.

EFFECTS OF INVENTION

According to the process of the present invention, the heat removalcapability of the polymerization vessel can be utilized effectivelyduring the polymerization period, and the polymerization time can beshortened. The quantity of polymerization initiator remaining within theproduct polymer can be reduced, and the generation of polymer scale inthe polymerization initiator supply pipe can be prevented, meaning aproduct with extremely low levels of fish eyes and initial discoloration(the property wherein discoloration occurs on molding of the polymer)can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1]

A diagram showing one example of a supply pipe installation of thepresent invention.

DESCRIPTION OF REFERENCE SYMBOLS

-   1 Polymerization vessel-   2 High-activity initiator supply pipe-   3 Water supply pipe-   4 Steam cleaning pipe-   5 to 9 Valves

BEST MODE OF CARRYING OUT THE INVENTION

As follows is a more detailed description of the present invention basedon a series of embodiments.

(A) Addition of High-Activity, Oil-Soluble Polymerization Initiator

In a suspension polymerization of vinyl chloride monomer or a monomermixture that includes vinyl chloride, generally, the polymerizationvessel is charged with an aqueous medium, a dispersant (a suspensionagent), a polymerization initiator, and various other additives asrequired, and hot water is then passed through the jacket to increasethe temperature inside the polymerization vessel to a predeterminedpolymerization reaction temperature, thereby initiating polymerization.By subsequently removing the heat generated by the polymerizationreaction using cooling devices such as a reflux condenser, thepolymerization reaction is allowed to proceed with the reaction mixtureinside the polymerization vessel maintained at a predetermined reactiontemperature. In the process of the present invention, the polymerizationreaction is typically conducted at a temperature of 35 to 75° C., andpreferably from 45 to 70° C.

A first characteristic feature of the present invention is the additionto the reaction mixture, for a specified time within the period fromcommencement of heat removal using the reflux condenser through tocompletion of the polymerization, of a high-activity, oil-solublepolymerization initiator (hereafter referred to as the high-activityinitiator), with a 10-hour half life temperature of no more than 40° C.at a concentration of 0.1 mol/L in benzene.

In this description, the half life of the polymerization initiatorrefers to the time required for the concentration of the polymerizationinitiator to fall to ½ of its initial value. The decomposition reactionof the polymerization initiator at any particular temperature can beconsidered a first-order reaction, so that at a certain temperature, arelationship represented by the following formula applies:log(a/x)=(k/2.303)t(wherein, x represents the concentration (mol/liter) of the initiator ata time t, a represents the initial concentration (same units) of theinitiator, k represents the decomposition rate constant determined bythe temperature, and t represents the time). By plotting a against t,the value of k at that temperature can be determined. By inserting thethus obtained value of k into the following formula:t_(1/2)(half life)=(ln2)/kthe half life at that temperature can be determined. The “10-hour halflife temperature at a concentration of 0.1 mol/L in benzene” used in thepresent invention refers to the temperature which results in a half lifeof 10 hours when the polymerization initiator is dissolved in benzene atthe specified concentration and the resulting solution is then sealedinside a container and allowed to stand at that temperature.

Examples of high-activity initiators for which the 10-hour half lifetemperature at a concentration of 0.1 mol/L in benzene is no more than40° C. include acetylcyclohexylsulfonyl peroxide (10-hour half lifetemperature at a concentration of 0.1 mol/L in benzene: 26.5° C.),isobutyryl peroxide (32.5° C.), α-cumyl peroxyneodecanoate (36.6° C.),diisopropylbenzene (36.4° C.), diallyl peroxycarbonate (38.3° C.), and3-hydroxy-1,1-dimethylbutyl peroxyneodecanoate (37° C.), and thesecompounds can be used either alone, or in combinations of two or moredifferent compounds. Of these high-activity initiators, isobutyrylperoxide, α-cumyl peroxyneodecanoate, and 3-hydroxy-1,1-dimethylbutylperoxyneodecanoate are preferred, and isobutyryl peroxide isparticularly desirable.

According to the present invention, the high-activity initiator is addedto the reaction mixture across a specified time period (hereafter alsoreferred to as the “high-activity initiator addition period”) within theperiod from commencement of heat removal using the reflux condenserthrough to completion of the polymerization. Heat removal using thereflux condenser is usually conducted so as not to cause rapid changesin the temperature inside the polymerization vessel or the jackettemperature, and to avoid any effects on foaming of the mixture insidethe polymerization vessel, meaning the quantity of heat removed isnormally increased gradually from the commencement of condenser heatremoval, through until a predetermined heat removal quantity is reached.Commencement of addition of the high-activity initiator must be noearlier than the commencement of heat removal using the refluxcondenser. In other words, addition may commence at the same time thatheat removal using the reflux condenser is commenced, immediately aftercommencement of heat removal, or at a specified time after thecommencement of heat removal. Commencement of the addition immediatelyafter the commencement of heat removal means within 10 minutes of thecommencement of heat removal. Addition of the high-activity initiator ispreferably commenced either simultaneously with the commencement of heatremoval, or immediately after the commencement of heat removal.Furthermore, although the addition of the high-activity initiator may becontinued until the completion of the polymerization, the addition ispreferably halted prior to the polymerization conversion rate reaching75%, and even more preferably prior to the polymerization conversionrate reaching 60%. Even if addition is continued once the polymerizationconversion rate has exceeded 75%, the polymerization time shorteningeffect generated by the additional quantity diminishes, making furtheraddition uneconomic.

In the present invention, the completion of the polymerization is thepoint at which the polymerization reaction is halted by addition, to themixture inside the polymerization vessel, of a sufficient quantity of apolymerization inhibitor (a material with polymerization inhibitingproperties) to halt the polymerization reaction. After this addition,the recovery of any unreacted monomer is usually commenced.

Those inhibitors typically used in vinyl chloride-based polymerproduction can be used as the polymerization inhibitor. Specificexamples of suitable inhibitors include phenol-based compounds such as2-t-butylphenol, 2-t-aminophenol, 2,4-dimethyl-6-tert-butylphenol,2,6-diisopropyl-p-cresol,n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,3,5-di-t-butyl-4-hydroxytoluene, triethyleneglycol-bis-[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],t-butylhydroxyanisole, t-butylhydroquinone,4,4′-butylidenebis(3-methyl-6-t-butylphenol),2,2′-methylene-bis(4-methyl-6-t-butylphenol),2,2′-methylene-bis(4-ethyl-6-t-butylphenol),2,6-di-t-butyl-4-methylphenol, 2,6-di-t-butyl-4-ethylphenol,2,6-di-t-butyl-4-hydroxymethylphenol,4,4′-methylenebis(2,6-di-t-butylphenol),4,4′-thiobis(6-t-butyl-m-cresol), andtetrakis[methylene-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate]methane;phosphorus compounds such as cyclicneopentanetetraylbis(octadecylphosphite); sulfur compounds such asdilauryl thiodipropionate, dimyristyl thiodipropionate, distearylthiodipropionate, alkali metal sulfates, alkali metal hydrogensulfites,and alkali metal thiosulfates; and nitrogen compounds such asN,N-diethylhydroxylamine and sodium nitrite. These polymerizationinhibitors can be used either alone, or in combinations of two or moredifferent compounds. Of the above compounds, from the viewpoints oflimiting scale adhesion to the polymerization vessel and achieving goodanti-initial discoloration for the product polymer,n-octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate,3,5-di-t-butyl-4-hydroxytoluene, triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate],2,2′-methylene-bis(4-ethyl-6-t-butylphenol),4,4′-butylidenebis(3-methyl-6-t-butylphenol), t-butylhydroxyanisole,t-butylhydroquinone, and N,N′-diethylhydroxylamine are preferred.

The quantity added of the polymerization inhibitor is typically within arange from 0.001 to 0.3 parts by mass, and preferably from 0.003 to 0.1parts by mass, and even more preferably from 0.005 to 0.05 parts bymass, per 100 parts by mass of the monomer or monomer mixture.

As a result of this characteristic feature (A), the heat removalcapability of the polymerization vessel can be utilized effectivelythroughout the polymerization time, and the polymerization time can beshortened. If addition of the high-activity initiator is commenced priorto commencement of heat removal using the reflux condenser, then thepolymerization reaction heat becomes excessively large and exceeds theheat removal capability of the polymerization vessel, making itimpossible to maintain the temperature inside the polymerization vesselat the predetermined temperature, and causing problems in the productpolymer such as an increase in the occurrence of fish eyes.

The quantity added of the high-activity initiator can be setappropriately in accordance with factors such as the quantity of anyother polymerization initiators added to the polymerization vessel afterpolymerization initiation, and the total heat removal capability of thepolymerization vessel being used, but is typically within a range from0.0001 to 0.2 parts by mass, and preferably from 0.001 to 0.1 parts bymass, per 100 parts by mass of the monomer or monomer mixture (the totalquantity of monomer supplied to the polymerization, including the vinylchloride monomer, and where used, any other copolymerizable monomers). Aquantity that falls within this range is preferred in terms of theinitial discoloration property of the product polymer, and thepolymerization time shortening effect achieved.

The rate of addition for the high-activity initiator can be setappropriately in accordance with factors such as the quantity of otherpolymerization initiators used after polymerization initiation, and thetotal heat removal capability of the polymerization vessel being used,but is typically within a range from 0.3 to 5% by mass, and preferablyfrom 0.5 to 3% by mass, of the entire quantity of the initiator perminute. If the rate of addition for the high-activity initiator is toohigh, then the quantity of heat generated by the polymerization reactionincreases rapidly, which can cause problems such as the heat removalcapability of the polymerization vessel becoming inadequate, making itimpossible to control the temperature inside the polymerization vesselat a constant value. Furthermore, if the rate of addition for thehigh-activity initiator is too low, then the unutilized fraction of thereflux condenser heat removal capability immediately following thecommencement of heat removal using the condenser increases, causing aworsening of the heat removal efficiency, and diminishing thepolymerization time shortening effect. Furthermore, the addition ratefor the high-activity initiator may be adjusted in accordance with theheat removal capability of the polymerization vessel. Provided thehigh-activity initiator is added essentially continuously throughout thehigh-activity initiator addition period, at an addition rate specifiedabove, then the actual mode of addition may be either continuous orintermittent. Furthermore, the physical state of the added high-activityinitiator may be either a solution generated by dilution in an organicsolvent, or an emulsion or suspension generated by dispersion of theinitiator in an aqueous medium, although a dispersion within an aqueousmedium is preferred. Aqueous emulsions are particularly preferred.

An aqueous emulsion of the high-activity initiator can be prepared bymechanically emulsifying the initiator and water. An emulsifying agentis usually used to improve the stability of the emulsion. Furthermore,in order to prevent freezing at low temperatures, a water-solublealcohol with a molecular weight of no more than 100 is preferably added.More specifically, the emulsion can be prepared, for example, by amethod in which a mixed liquid, formed by dissolving or dispersing anemulsifying agent and a water-soluble alcohol with a molecular weight ofno more than 100 in water, is added to the high-activity initiator withstirring, or a method in which the initiator is added to the aqueoussolution. The emulsifying agent may also be dissolved in thehigh-activity initiator.

In the preparation of an aqueous emulsion of the high-activityinitiator, a high-activity initiator that has been diluted with adiluent may also be used. Examples of suitable diluents include organicsolvents such as benzene, toluene, and aliphatic hydrocarbons, as wellas plasticizers such as dimethyl phthalate and dioctyl phthalate. Thequantity of the diluent typically accounts for no more than 40% by massof the aqueous emulsion.

The quantity of the high-activity initiator within the aqueous emulsiondescribed above is typically from 5 to 80% by mass, and for practicalpurposes, is preferably from 10 to 65% by mass.

The water-soluble alcohol added to prevent freezing is preferably analcohol with a molecular weight of no more than 100. Examples ofsuitable alcohols include methyl alcohol, ethyl alcohol, n-propylalcohol, isopropyl alcohol, ethylene glycol, and glycerol. Theconcentration of the water-soluble alcohol within the aqueous emulsionis typically within a range from 2% by mass to 30% by mass, althoughconcentrations exceeding 10% by mass are preferred, as they enable thefreezing point of the aqueous emulsion to be lowered to −25° C. orlower, and enable a low viscosity to be achieved at low temperatures.

The emulsifying agent used in the preparation of the aqueous emulsionmay be one or more emulsifying agents selected from the group consistingof anionic surfactants, cationic surfactants, non-ionic surfactants, anddispersants, provided the agent does not effect the physical propertiesof the product polymer. Of these emulsifying agents, the use of acombination of a non-ionic surfactant and a dispersant described belowis the most preferred, as such combinations produce excellent stabilityof the high-activity initiator aqueous emulsion, and also yieldfavorable properties for the product polymer.

The above non-ionic surfactant is preferably a polyhydric alcohol esterof a fatty acid of 10 to 22 carbon atoms, and suitable examples includethe various mono-, di-, and tri-sorbitan fatty acid esters, the variousmono-, di-, tri-, tetra-, and poly saccharose fatty acid esters, thevarious mono-, di-, and tri-glycerol fatty acid esters, the variousmono- and di-propylene glycol fatty acid esters, the various mono-, di-,tri-, tetra-, and poly sorbitol fatty acid esters, and the variousmono-, di-, tri-, and tetra-pentaerythritol fatty acid esters. Here, theterm “fatty acid” refers to a fatty acid such as oleic acid, lauricacid, palmitic acid, or stearic acid. A polyhydric alcohol fatty acidester using one or more of these fatty acids is used, and if blending isconducted so that the HLB value of the surfactant mixture is within arange from 1 to 10, then stable, fine emulsified particles are generatedwithin the aqueous emulsion. The quantity added of the non-ionicsurfactant typically accounts for 0.01 to 10% by mass of thehigh-activity initiator aqueous emulsion. Quantities from 0.1 to 5% bymass are preferred. Specific examples of suitable commercially availablenon-ionic surfactants include Monogly MB, Nonion OP-80R, and NonionOP-85R manufactured by NOF Corporation, and Rikemal PO-100 and Rikemal0-71-D manufactured by Riken Vitamin Co., Ltd.

Furthermore, suitable examples of the aforementioned dispersant includeguar gum, locust bean gum, Abelmoschus manihot, tragacanth gum, gumarabic, viscose, methyl cellulose, ethyl cellulose, hydroxyethylcellulose, carboxymethyl cellulose, soluble starch, carboxymethylstarch, dialdehyde starch, polyvinyl alcohol, sodium polyacrylate, andpartially or completely saponified polyvinyl acetate. One or more ofthese dispersants can be used.

The preparation of the aqueous emulsion of the high-activity initiatorcan be conducted using conventional apparatus. For example, paddle-type,propeller-type, or turbine-type mechanical rotational mixers, colloidmills, homogenizers, high-speed shear devices, line mixers, andultrasound homogenizers can be used.

(B) Addition of Water

A second characteristic feature of the present invention is thecontinuous or intermittent addition of water to the reaction mixturethrough the supply pipe for the polymerization initiator, at leastthroughout the aforementioned high-activity initiator addition period(namely, the period from addition commencement through to additioncompletion).

Examples of the water that is used include deionized water, tap water,industrial water, and soft water, although deionized water is preferred.

The period over which the water is added may be any period that includesthe period during which the high-activity initiator is supplied. Inother words, addition of the water may be commenced prior to thecommencement of addition of the high-activity initiator, and may becontinued after completion of the high-activity initiator addition. Thewater may also be added continuously from initiation through tocompletion of the polymerization. From the viewpoint of preventing scaleadhesion inside, and in the region around the outlet of, thehigh-activity initiator supply pipe, the addition of the water ispreferably commenced prior to the commencement of heat removal using thereflux condenser, and is preferably continued for at least 10 minutesafter the completion of supply of the high-activity initiator, beforebeing halted.

There are no particular restrictions on the rate of water addition,although from the viewpoint of ensuring favorable fish eye reduction andscale adhesion prevention effects, the flow rate of the water ispreferably at least 200 g/min·cm², and even more preferably 400g/min·cm² or greater, through a cross-section of the high-activityinitiator supply pipe. If this flow rate is too small, then the adhesionof scale inside, and in the region around the outlet of, thehigh-activity initiator supply pipe becomes more likely. This results inan increased likelihood of certain problems, including blockage of thepipe as the number of batches accumulates, and an increase in fish eyesin the product polymer caused by adhered scale breaking away andbecoming incorporated within the polymer. There are no particularrestrictions on the upper limit for the flow rate, although forpractical reasons, the flow rate is typically no more than 3,000g/min·cm², and is preferably 1,000 g/min·cm² or less.

There are no particular restrictions on the quantity of water added,although in order to ensure that the fill ratio of the reaction mixtureinside the polymerization vessel, that is, the ratio of the volume ofthe liquid reaction mixture relative to the internal volume of thepolymerization vessel, does not become overly large, the quantity addedis preferably no more than the volumetric shrinkage associated with theprogression of the polymerization reaction. The above fill ratio ispreferably maintained within a range from 60 to 90%, and is even morepreferably held within a range from 70 to 90%.

There are no particular restrictions on the temperature of the water,although the temperature is preferably within a range from 5 to 50° C.,and even more preferably from 10 to 40° C.

(C) Cleaning of the High-Activity Initiator Supply Pipe Using Steam

A third characteristic feature of the present invention is the passageof steam through the aforementioned high-activity initiator supply pipefollowing completion of the addition of water, thereby decomposing andcleaning any residual high-activity initiator, either near the pointwhere the supply pipe exits into the polymerization vessel, or in anyother location where there is a possibility of the pipe making contactwith gaseous monomer.

This cleaning operation using steam is preferably conducted immediatelyfollowing completion of the water addition. There are no particularrestrictions on the time of this steam cleaning operation, althoughtypically the time is within a range from 10 to 180 seconds, andpreferably from 20 to 120 seconds. If the cleaning time is too short,then the scale adhesion prevention effect is inadequate. In contrast, ifthe cleaning time is too long, the high-activity initiator supply pipecan become overheated, which can cause degradation of initiator locatedoutside those regions where the pipe contacts the gaseous monomer, andcan also cause a deterioration in stability.

There are no particular restrictions on the steam flow rate, although inorder to ensure satisfactory cleaning of the high-activity initiatorsupply pipe, the flow rate is preferably at least 1,000 g/min·cm²through a cross-section of the supply pipe. There are no particularrestrictions on the upper limit for this flow rate, although forpractical reasons, the flow rate is typically no more than 7,000g/min·cm²

Provided satisfactory cleaning of the high-activity initiator supplypipe can be achieved, the steam may be either saturated water vapor orsuperheated water vapor.

There are no particular restrictions on the temperature of the steam,provided the resulting scale adhesion prevention effect is satisfactory.The temperature of the steam inside the pipe (the steam cleaning pipe)used for supplying the steam to the high-activity initiator supply pipeis preferably at least 120° C., and is even more preferably 130° C. orhigher. Furthermore, for practical reasons, the temperature is typicallyno more than 280° C.

There are no particular restrictions on the steam pressure, provided theresulting scale adhesion prevention effect is satisfactory. The pressureinside the aforementioned steam cleaning pipe, measured when supply ofthe steam to the high-activity initiator supply pipe has been halted, ispreferably within a range from 0.1 to 3 MPa·G,and even more preferablyfrom 0.1 to 1 MPa·G.

In order to ensure that the scale adhesion prevention effect provided bythe steam manifests satisfactorily, the inner surface temperature of theportion of the high-activity initiator supply pipe between the point ofintersection with the steam cleaning pipe and the pipe outlet into thepolymerization vessel is preferably at least 70° C., and is even morepreferably 80° C. or higher. Furthermore, for practical reasons, thisinner surface temperature is typically no more than 200° C.

-Supply Pipe and Steam Pipe Installations-

In the supply pipe installation for the high-activity initiator and thewater, the water is preferably able to be added continuously orintermittently to the reaction mixture inside the polymerization vesselthrough the high-activity initiator supply pipe. Furthermore, the steampipe installation is preferably constructed so that the high-activityinitiator supply pipe is able to be cleaned using the steam. One exampleof a supply pipe installation of the present invention is shown inFIG. 1. Those elements such as the stirrer and reflux condenser, whichare not directly related to this description, have been omitted. In FIG.1, a water supply pipe 3 is interconnected with a high-activityinitiator supply pipe 2. As a result, the water can be addedcontinuously or intermittently to the reaction mixture inside thepolymerization vessel 1 via the high-activity initiator supply pipe 2.Furthermore, a steam cleaning pipe 4 is also interconnected with thehigh-activity initiator supply pipe 2. As a result, the high-activityinitiator supply pipe 2 can be subjected to steam cleaning.

-Other Conditions-

There are no particular restrictions on the other conditions associatedwith the process of the present invention, and the conditions employedin conventional aqueous suspension polymerization reactions of eithervinyl chloride monomer, or a monomer mixture comprising vinyl chlorideas the primary component, can be used.

-Polymerization Vessel

A polymerization vessel used in conducting the process of the presentinvention comprises a reflux condenser as a cooling device. Thepolymerization vessel usually also comprises a jacket for heating andcooling, and although not essential, preferably also comprises a baffleor a coil or the like with cooling capabilities.

Furthermore, the polymerization vessel may also comprise a so-calledexternal heat exchange device, in which the mixed slurry inside thepolymerization vessel is extracted with a pump, passed through a heatexchanger disposed outside the polymerization vessel, and then returnedto the polymerization vessel.

Heat removal by the reflux condenser is commenced once the temperatureof the mixture inside the polymerization vessel has reached apredetermined polymerization reaction temperature, and in order not tocause rapid changes in the temperature of the mixture inside thepolymerization vessel or the jacket temperature, the quantity of heatremoved is increased gradually until a predetermined heat removalquantity is reached, and this heat removal quantity is then maintained.The time required from the commencement of heat removal using the refluxcondenser until the predetermined heat removal quantity is reached istypically from 5 to 120 minutes, and preferably from 15 to 60 minutes.Furthermore, after the predetermined heat removal quantity has beenreached, the quantity of heat removed by the reflux condenser may stillbe altered at points during the polymerization.

Furthermore, in order to prevent a deterioration in product quality as aresult of using the reflux condenser, further addition of suspensionagents or antifoaming agents, or adjustment of the rotational speed ofthe stirring blade inside the polymerization vessel, may also beconducted.

-Monomer

The monomer used in the present invention is either vinyl chloride or amonomer mixture comprising vinyl chloride as the primary component. Amonomer mixture comprising vinyl chloride as the primary componentcomprises at least 50% by mass of vinyl chloride, as well as anothermonomer which is copolymerizable with the vinyl chloride. Examples ofother monomers which are copolymerizable with vinyl chloride includevinyl esters such as vinyl acetate and vinyl propionate; acrylate estersor methacrylate esters such as methyl acrylate and ethyl acrylate;olefins such as ethylene and propylene; as well as other monomers suchas maleic anhydride, acrylonitrile, styrene and vinylidene chloride.These monomers can be used either alone, or in combinations of two ormore different monomers.

-Polymerization Initiator

There are no particular restrictions on the polymerization initiatorused for initiating the polymerization in a process of the presentinvention, and the types of initiators used in conventional vinylchloride-based polymer production are suitable. Specific examples ofthese polymerization initiators include peroxycarbonate compounds suchas diisopropyl peroxydicarbonate, di-2-ethylhexyl peroxydicarbonate,diethoxyethyl peroxydicarbonate, and di-sec-butyl peroxydicarbonate;peroxy ester compounds such as t-butyl peroxypivalate, t-hexylperoxypivalate, t-butyl peroxyneodecanoate, α-cumyl peroxyneodecanoate,t-butyl peroxyneoheptanoate, t-amyl peroxyneodecanoate, and3-hydroxy-1,1-dimethylbutyl peroxyneodecanoate; and peroxides such asisobutyryl peroxide, acetylcyclohexylsulfonyl peroxide,2,4,4-trimethylpentyl-2-peroxyphenoxy acetate, and3,5,5-trimethylhexanoyl peroxide. These polymerization initiators can beused either alone, or in combinations of two or more differentinitiators. The quantity used is typically within a range from 0.01 to 1part by mass, and preferably from 0.02 to 0.2 parts by mass, per 100parts by mass of the monomer raw material. These polymerizationinitiators may be added either in solution form, having been dilutedwith an organic solvent, or in the form of an emulsion or suspensionformed by dispersion within an aqueous medium. An aqueous emulsion ispreferred. Preparation of an aqueous emulsion of the polymerizationinitiator can be achieved in the same manner as described for theaqueous emulsion of the high-activity initiator.

-Suspension Stabilizer

There are no particular restrictions on the suspension stabilizer usedwhen polymerizing either vinyl chloride, or a monomer mixture comprisingvinyl chloride, in an aqueous medium in accordance with a process of thepresent invention, and the types of stabilizers used in conventionalvinyl chloride-based polymer production are suitable. Specific examplesof these suspension stabilizers include water-soluble cellulose etherssuch as methyl cellulose, hydroxyethyl cellulose, hydroxypropylcellulose and hydroxypropylmethyl cellulose; partially saponifiedpolyvinyl alcohols such as water-soluble partially saponified polyvinylalcohol and oil-soluble partially saponified polyvinyl alcohol; andwater-soluble polymers such as acrylic acid polymers and gelatin. Thesestabilizers can be used either alone, or in combinations of two or moredifferent stabilizers. The total quantity added of these suspensionstabilizers is typically set appropriately within a range from 0.02 to 1part by mass per 100 parts by mass of the monomer.

-Other Additives

In the process of the present invention, if required, other additivestypically used in the production of vinyl chloride-based polymers, suchas polymerization degree regulators, chain transfer agents, andantistatic agents and the like may also be used.

Other conditions associated with the polymerization, such as the methodof supplying the aqueous medium, the vinyl chloride or monomer mixturecomprising vinyl chloride, the suspension stabilizer, and thepolymerization initiator and the like to the polymerization vessel, aswell as the relative proportions added and the polymerization degree,may be similar to conventional processes.

As follows is a more detailed description of the process of the presentinvention using a series of examples and a comparative example.

The polymerization conversion rate at any specific point during apolymerization conducted in either of the examples or the comparativeexample was determined by actually conducting the polymerization underthe specified polymerization conditions, adding an antioxidant to thepolymerization system at a specific time to halt the polymerization,measuring the quantity of polymer produced at that point, and then usingthis measured quantity to calculate the polymerization conversion rateat that point. A plurality of polymerization reactions were conductedunder the same conditions, with the time until the polymerization washalted extended by 0.5 hours in each successive reaction, and thepolymerization conversion rate was measured in each case. This methodwas used to establish, in advance, the relationship between reactiontime and the polymerization conversion rate for each of the specificreaction conditions, and the polymerization conversion rate at anyparticular time was then specified on the basis of this relationship.

EXAMPLE 1

The following description is based on FIG. 1. With the valves 5 to 9closed, a polymerization vessel 1 of internal capacity 2 m³, fitted witha reflux condenser, was charged with 856 kg of deionized water, 206 g ofa partially saponified polyvinyl alcohol with a saponification degree of79.5 mol %, and 137 g of hydroxypropylmethyl cellulose with a methoxysubstitution degree of 28.5% by mass and a hydroxypropoxy substitutiondegree of 8.9% by mass. Subsequently, the inside of the polymerizationvessel 1 was degassed until the internal pressure reached 8 kPa, and 685kg of vinyl chloride monomer was then added. With the mixture undergoingconstant stirring, 760 g of a 50% by mass aqueous emulsion ofdi-2-ethylhexyl peroxydicarbonate (10-hour half life temperature at aconcentration of 0.1 mol/L in benzene: 43.4° C.) was added as apolymerization initiator (1), while a temperature raising process wasstarted by passing hot water through the jacket, and when thetemperature inside the polymerization reaction vessel 1 reached 57.0°C., the polymerization was allowed to proceed with the temperaturemaintained at that level. When the temperature inside the polymerizationvessel 1 reached 57° C., the valves 6 to 8 were opened, and the additionof deionized water was commenced from the water supply pipe 3, using aflow rate of 500 g/min·cm² through a cross-section of the high-activityinitiator supply pipe 2. When the polymerization conversion rate reached15%, heat removal using the reflux condenser was commenced, and thevalve 5 was opened, thereby commencing addition to the polymerizationvessel 1, from the high-activity initiator supply pipe 2, of a 15% bymass aqueous emulsion of isobutyryl peroxide (half life temperature asabove: 32.5° C.) as a polymerization initiator (2). The quantity addedof the 15% by mass aqueous emulsion of isobutyryl peroxide was 400 g,and the rate of addition was 6.7 g per minute. After addition of thepredetermined quantity of isobutyryl peroxide, the valve 5 was closed,thereby halting addition of the isobutyryl peroxide. The polymerizationconversion rate at this point was 40%. 10 minutes after halting theaddition of the isobutyryl peroxide, the valves 6 and 8 were closed,thereby halting the addition of the deionized water. Subsequently, steamat a temperature of 240° C., which exhibited a pressure inside the steamcleaning pipe 4 with the valve 9 closed of 0.3 MPa·G, was introducedinto the high-activity initiator supply pipe 2 by opening the valve 9,and after steam cleaning for 30 seconds, the valves 7 and 9 were closed.The polymerization reaction was then continued, and when the pressureinside the polymerization vessel 1 had fallen to 0.588 MPa·G, 294 g of a35% by weight aqueous emulsion of triethyleneglycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate] was addedas a polymerization inhibitor to halt the polymerization reaction, theunreacted monomer was recovered, the reaction mixture slurry wasextracted from the polymerization vessel, and this slurry was thendewatered and dried, yielding a vinyl chloride-based polymer.

Following washing of the inside of the polymerization vessel with water,5 consecutive batches were conducted using the same procedure describedabove. The level of fish eyes in the polymer produced from the finalbatch was measured using the method described below. The results areshown in Table 1. Furthermore, the polymerization time for the finalbatch, and the level of scale adhesion in the vicinity of the outlet ofthe high-activity initiator supply pipe 2 following completion of thefinal batch are also recorded in Table 1.

[Fish Eyes]

25 g of a mixture obtained by mixing together 100 parts by mass of thesample polymer, 50 parts by mass of dioctyl phthalate, 0.5 parts by massof tribasic lead sulfate, 1.5 parts by mass of lead stearate, 0.1 partsby mass of titanium oxide, and 0.05 parts by weight of carbon black waskneaded for 5 minutes at 140° C. using a six inch kneading roll mill.The mixture was then molded into a sheet of width 10 mm and thickness0.2 mm, and the number of transparent spots (fish eyes) per 100 cm² ofthe sheet was counted and recorded.

EXAMPLE 2

With the exceptions of altering the quantity added and the rate ofaddition of the isobutyryl peroxide aqueous emulsion of thepolymerization initiator (2), and altering the polymerization conversionrate at the point where reflux condenser heat removal was commenced asshown in Table 1, a vinyl chloride-based polymer was produced in thesame manner as the example 1, and then subjected to the samemeasurements. The results are shown in Table 1.

COMPARATIVE EXAMPLE 1

With the exceptions of not conducting the continuous water addition, northe steam cleaning of the pipes, a vinyl chloride-based polymer wasproduced in the same manner as the example 1, and then subjected to thesame measurements. The results are shown in Table 1. TABLE 1 ComparativeExample 1 Example 2 example 1 Polymerization 760 760 760 initiator (1) gg g Polymerization 400 800 400 initiator (2) g g g Continuous flow rate500 500 none of deionized water (g/min · cm²) Polymerization 15 10 15conversion rate (%) when reflux condenser heat removal commenced Rate ofaddition of 6.7 6.7 6.7 polymerization g/minute g/minute g/minuteinitiator (2) Polymerization 40 60 40 conversion rate (%) when additionof polymerization initiator (2) completed Polymerization time 4 hours 163 hours 49 4 hours 17 minutes minutes minutes Fish eyes (number) 8 10 35State of scale adhesion Almost no Almost no Adhesion of scale adhesionscale adhesion hard scale(Notes)Polymerization intiator (1): 50% by mass aqueous emulsion ofdi-2-ethylhexl peroxycarbonatePolymerization initiator (2): 15% by mass aqueous emulsion of isobutyrylperoxide.

1. A process for producing a vinyl chloride-based polymer, wherein asuspension polymerization of either vinyl chloride monomer, or a mixtureof vinyl chloride monomer and another copolymerizable monomer, isconducted in a polymerization vessel fitted with a reflux condenser,said process comprising: (A) adding to a reaction mixture ahigh-activity, oil-soluble polymerization initiator, with a 10-hour halflife temperature of no more than 40° C. at a concentration of 0.1 mol/Lin benzene, for a specified time within a period from commencement ofheat removal using said reflux condenser through to completion ofpolymerization, (B) adding water either continuously or intermittentlyto said reaction mixture through a supply pipe for said polymerizationinitiator, at least during a period from commencement of addition ofsaid high-activity, oil-soluble polymerization initiator through tocompletion of said addition, and (C) passing steam through said supplypipe following completion of addition of said water.
 2. The processaccording to claim 1, wherein a quantity added of said high-activity,oil-soluble polymerization initiator is within a range from 0.0001 to0.2 parts by mass per 100 parts by mass of said monomer or monomermixture.
 3. The process according to either claim 1, wherein a rate ofaddition of said high-activity, oil-soluble polymerization initiator iswithin a range from 0.3 to 5% by mass of an entire quantity of saidinitiator per minute.
 4. The process according to claim 1, wherein awater flow rate is at least 200 g/min·cm² through a cross-section ofsaid supply pipe.
 5. The process according to claim 1, wherein additionof said high-activity, oil-soluble polymerization initiator is commencedeither simultaneously with commencement of heat reduction using saidreflux condenser, or within 10 minutes of said commencement of heatreduction, and is completed prior to a polymerization conversion ratereaching 75%.
 6. The process according to claim 1, wherein saidcopolymerizable monomer is at least one selected from the groupconsisting of vinyl esters, acrylate esters, methacrylate esters,olefins, maleic anhydride, acrylonitrile, styrene, and vinylidenechloride.
 7. The process according to claim 1, wherein saidhigh-activity, oil-soluble polymerization initiator is at least oneselected from the group consisting of acetylcyclohexylsulfonyl peroxide,isobutyryl peroxide, α-cumyl peroxyneodecanoate, diisopropylbenzene,diallyl peroxycarbonate, and 3-hydroxy-1,1-dimethylbutylperoxyneodecanoate.
 8. The process according to claim 1, wherein atemperature of said steam is at least 120° C.